1. Field of the Invention
The present invention relates to a radiographic intensifying screen, and more particularly to a radiographic intensifying screen which is improved in the antistatic properties.
2. Description of the Prior Art
In various kinds of radiography such as medical radiography for diagnosis and industrial radiography for nondestructive inpsection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film to enhance a radiographic speed of the radiography.
The radiographic intensifying screen basically comprises a support and a phosphor layer provided on one surface of the support. A transparent film made of a plastic material is generally provided on a free surface of the phosphor layer (surface not facing the support) to keep the phosphor layer from chemical and physical deterioration.
The phosphor layer comprises a binder and phosphor particles dispersed therein. The phosphor particles emit a light of high luminance when excited with a radiation such as X-rays. Accordingly, the phosphor particles give light emission of high luminance in proportion to the dose of the radiation having passed through an object, and the radiographic film placed in close contact with the phosphor layer of the screen can be sufficiently sensitized by the emitted light to form a radiation image of the object thereon, even if the radiation is applied to the object at a relatively small dose.
In a practical use of the radiographic intensifying screen in the radiography, a pair of screens are combined with a radiographic film in a cassette in such a manner that the screens are brought into close contact with both surfaces of the radiographic film. Since the cassette is light-blocking, the radiographic film can be prevented from being exposed to a light even in a light room, and the screen can be readily brought into close contact with the radiographic film.
However, since both of the screen and the film are made of plastic materials, an electrostatic charge is liable to occur by rubbing of the screen with the film when the film is inserted into or taken out of the cassette, and thereby the surfaces of the screen and the film are easily electrified to give an electrostatic discharge between the screen and the film. Because of the electrostatic discharge, the radiographic film is sensitized and the resulting image on the developed film suffers a noise (i.e., static mark) to reduce the precision in the medical diagnosis.
Recently, a continuous radiographic system not using a cassette (cassette-less sytem) has been developed and is now practically utilized for enhancing the radiographic speed. For example, in a radiographic apparatus for blood vessel (i.e., AOT system), a pair of radiographic intensifying screens are fixed in the predetemined position, and a number of radiographic films are set in a magazine. For conducting the radiographic operation, the radiographic films having been set in the magazine are automatically and continuously transferred and inserted between the two screens one after another. The used film is then transferred and received in other magazine for used films, and at the same time other unused film is inserted between the screens.
In the above-mentioned cassette-less system, the radiographic film is liable to be much more charged than the case of using a cassette. That is, the film easily electrified by the static charge caused by rubbing with another film when the film is taken out of the magazine or rubbing with transferring members in the transferring procedure, as well as the contact with the screens.
For preventing the occurrence of the static electrification of the radiographic film, various measures have been proposed and utilized. For example, a method of coating or spraying a liquid antistatic agent (e.g., commercially available Antistatic cleaner) onto a radiographic intensifying screen is generally carried out to reduce electrification of the film. However, this method only forms a coated layer of the antistatic agent on the surface of the screen, so that the coated layer tends to gradually separate from the screen with time after repeated contact with the radiographic film, etc., and hence the screen is hardly used for a long period of time. Especially in a high-speed radiographic system involving a number of radiographic operations and repeated usage of the screen, there occurs such necessity that the antistatic treatment (coating or spraying the antistatic agent) should be repeatedly done at appropriate intervals.
As other measures for preventing electrification, a method of imntroducing antistatic property into a radiographic intensifying screen per se has been proposed. For example, a carbon black layer is provided between the support and the phosphor layer and an antistatic agent is incorporated into the protective film, as disclosed in U.S. Pat. No. 4,090,085 to Shimiya et al.